Kim, Hojung; (2020) The Paleocene radiation of calcareous nannoplankton. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The Cretaceous-Paleogene (K-Pg) mass extinction (66 million years ago, Mya)eliminated over 70% of marine biota (Raup and Sepkoski, 1982) and the coccolithophores (or nannoplankton), one of the key marine phytoplankton groups, lost more than 90% of existing species. Their recovery in the early Danian was an important part of rebuilding of the marine ecosystem both in increasing biodiversity but also restoring ecosystem function, and, especially, the production and export of organic carbon. This study focuses on the record of coccolithophore lith and cell size because one of the most striking characteristics of the incoming Danian coccolithophore species is their unusually small size, producing coccoliths of around 1-3 μm compared with the extinct Cretaceous taxa with average coccolith size of 8 μm. By recording the evolution of coccolithophore lith and cell sizes in the post-extinction ocean, as well as the diversity and disparity, we aim to capture the temporal and special recovery pattern of the coccolithophore community and better understand their role in marine ecosystem recovery, especially in the return of the biological pump efficiency (Chapter 5). The study provides a 10 My Paleocene record of lith and cell sizes after the KPg with a particular emphasis on Danian interval (Chapter 4). Data was collected at high resolution for the Danian (20-30 Ky for the lower Danian and 50-60 Ky for the upper Danian) and lower resolution for the remainder of the Paleocene (every 100-200 Ky), using materials from the North Atlantic (IODP sites 1403 and 1407, Newfoundland Ridge) and North Pacific (ODP Site 1209, Shatsky Rise). As cellular traits (i.e., diameter and volume) cannot be directly measured from the most common type of fossil (liths), we developed a relatively easy and simple method to transform lith data into cell size data (Chapter 3). Part of the challenges in studying the Danian coccolithophores has been the Danian taxonomy, which has brought confusions among the users for many years due to their particularly small size. Producing consistent and reproducible taxonomy, therefore, is important and we address this issue (especially within the Prinsiaceae family) and provide reviews on the Danian taxonomy including 2 new species we describe from this study (Chapter 2). These morphometric data show that nannoplankton community cell size increased rapidly in the immediate aftermath of the mass extinction with recovery taking ~0.75 My. The records indicate that this cell and lith size increase occurred in several important steps during the earliest Danian and then more gradually through the rest of the Paleocene, although the rates and the timings are taxon specific. Nannoplankton diversity recovery (i.e., species richness) took far longer to regain pre-extinction levels (~ +8 My). The increase in community cell size and diversity likely were significant contributing factors in restoration of the biological pump around +1.8 My. Increasing species richness and disparity likely reflected expansion into new niches with increased seasonal range, and the broader cell size spectrum increased the diversity of food supply and would have enhanced export efficiency. Overall, our research reveals that the nannoplankton community evolution played a critical role in restoration of the biological pump and expansion of marine food-web. Based on our result, together with other environmental indicators (e.g. appearance of foraminifera photosymbiosis and oligotrophic nannoplankton species), the marine ecosystem regained its function to pre-extinction level by the end of the Danian.

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